Nina Fechler

Controlled folding of synthetic polymer chains through the formation of positionable covalent bridges

Covalent bridges play a crucial role in the folding process of sequence-defined biopolymers. This feature, however, has not been recreated in synthetic polymers because, apart from some simple regular arrangements (such as block co-polymers), these macromolecules generally do not exhibit a controlled primary structure—that is, it is difficult to predetermine precisely the sequence of their monomers. Herein, we introduce a versatile strategy for preparing foldable linear polymer chains. Well-defined polymers were synthesized by the atom transfer radical polymerization of styrene. The controlled addition of discrete amounts of protected maleimide at precise times during the synthesis enabled the formation of polystyrene chains that contained positionable reactive alkyne functions. Intramolecular reactions between these functions subsequently led to the formation of different types of covalently folded polymer chains. For example, tadpole (P-shaped), pseudocyclic (Q-shaped), bicyclic (8-shaped) and knotted (α-shaped) macromolecular origamis were prepared in a relatively straightforward manner. Synthetic polymers are typically difficult to fold into particular origamis because the monomers can usually not be precisely organized along their backbones. Reactive alkyne groups have now been placed at specific locations in linear polystyrene chains, enabling those to be folded into predetermined shapes through intramolecular covalent bonding.

Salt melt synthesis of ceramics, semiconductors and carbon nanostructures.

Materials synthesis in the liquid phase, or wet-chemical synthesis, utilizes a solution medium in which the target materials are generated from a series of chemical and physical transformations. Although this route is central in organic chemistry, for materials synthesis the low operational temperature range of the solvent (usually below 200 °C, in extreme 350 °C) is a serious restriction. Here, salt melt synthesis (SMS) which employs a molten inorganic salt as the medium emerges as an important complementary route to conventional liquid phase synthesis. Depending on the nature of the salt, the operational temperature ranges from near 100 °C to over 1000 °C, thus allowing the access to a broad range of inorganic crystalline materials and carbons. The recent progress in SMS of inorganic materials, including oxide ceramic powders, semiconductors and carbon nanostructures, is reviewed here. We will introduce in general the range of accessible materials by SMS from oxides to non-oxides, and discuss in detail based on selected examples the mechanisms of structural evolution and the influence of synthetic conditions for certain materials. In the later sections we also present the recent developments in SMS for the synthesis of organic solids: covalent frameworks and polymeric semiconductors. Throughout this review, special emphasis is placed on materials with nanostructures generated by SMS, and the possible modulation of materials structures at the nanoscale in the salt melt. The review is finalized with the summary of the current achievements and problems, and suggestions for potential future directions in SMS.

Metal-free Ionic Liquid-derived Electrocatalyst for High-Performance Oxygen Reduction in Acidic and Alkaline Electrolytes

We report herein a new simplified approach to metal-free N-doped carbon aerogels made from ionic liquids by a bottom-up “salt templating” strategy, which are known to be promising electrocatalytic materials. An optimized pore transport system as well as a high catalytically active surface allowed the ionic liquid-derived carbons to possess favourably high electrocatalytic performance in oxygen reduction under alkaline and, not found for similar materials made by other pathways, under technically more relevant acidic conditions. Even in acid, the performance favourably compares with a commercial Pt-based catalyst.

Polymerization under Hypersaline Conditions: A Robust Route to Phenolic Polymer‐Derived Carbon Aerogels

Polymer-derived carbon aerogels can be obtained by direct polymerization of monomers under hypersaline conditions using inorganic salts. This allows for significantly increased mechanical robustness and avoiding special drying processes. This concept was realized by conducting the polymerization of phenol-formaldehyde (PF) in the presence of ZnCl2 salt. Afterwards, the simultaneous carbonization and foaming process conveniently converts the PF monolith into a foam-like carbon aerogel. ZnCl2 plays a key role, serving as dehydration agent, foaming agent, and porogen. The carbon aerogels thus obtained are of very low density (25 mg cm-3 ), high specific surface area (1340 m2  g-1 ), and have a large micro- and mesopore volume (0.75 cm3  g-1 ). The carbon aerogels show very promising potential in the separation/extraction of organic pollutants and for energy storage.

Eutectic Syntheses of Graphitic Carbon with High Pyrazinic Nitrogen Content.

Mixtures of phenols/ketones and urea show eutectic behavior upon gentle heating. These mixtures possess liquid-crystalline-like phases that can be processed. The architecture of phenol/ketone acts as structure-donating motif, while urea serves as melting-point reduction agent. Condensation at elevated temperatures results in nitrogen-containing carbons with remarkably high nitrogen content of mainly pyrazinic nature.

Vanadium nitride@N-doped carbon nanocomposites: tuning of pore structure and particle size through salt templating and its influence on supercapacitance in ionic liquid media

The facile one-step synthesis of composites of highly porous carbons with functional metal nitride nanoparticles with tunable surface area, pore size, pore volume and nanoparticle size is presented using simple salts as porogens. Vanadium nitride (VN) nanoparticles in high surface area nitrogen-doped carbons are made by a simple heat-treatment of mixtures consisting of a vanadium precursor (VOCl3 or NH4VO3), the ionic liquid 1-ethyl-3-methyl-imidazolium dicyanamide (Emim-dca) as solvent and the nitrogen/carbon source, and salts, i.e. cesium or zinc acetate, as porogens. The synthesis takes advantage of a homogeneous starting solution, while in situ pore generation is obtained through demixing in later stages of the reaction. As compared to other templates such as silica, the salt is easily removed with water. Furthermore, the composite properties can conveniently be controlled by the variation of the salt nature and composition of the precursor mixture. Cesium acetate as a porogen at low concentrations results in microporous materials with small VN nanoparticles with a surface area of around 1000 m2 g−1, while increasing salt amounts promote small mesopores with bigger nanoparticles and surface areas of up to 2400 m2 g−1. The utilization of zinc acetate enables the synthesis of entirely mesoporous composites with very small vanadium nitride nanoparticles and surface areas of 800 m2 g−1. Mixtures of these two salt porogens give access to independently tunable surface area, pore size, pore volume and particle size. Comparative electrochemical testing in two ionic liquid (IL) electrolytes quantifies the accessibility of the surface area in two systematic sample series and indicates optimized surface access even for large electrolytes. A variation of ion radius in similar IL-systems quantifies the accessibility of surface in the different hybrid materials. Optimal energy density of the composites in supercapacitor electrodes can only be realized in a fine balance of charge density and electronic/ionic conductivity, which is here realized by fine-tuning the structural parameters.

“Caffeine Doping” of Carbon/Nitrogen-Based Organic Catalysts: Caffeine as a Supramolecular Edge Modifier for the Synthesis of Photoactive Carbon Nitride Tubes

An alternative method for the structure tuning of carbon nitride materials by using a supramolecular approach in combination with caffeine as lining‐agent is described. The self‐assembly of the precursor complex consisting of melamine and cyanuric acid can be controlled by this doping molecule in terms of morphology, electronic, and photophysical properties. Caffeine is proposed to insert as an edge‐molecule eventually leading to hollow tube‐like carbon nitride structures with improved efficiency of charge formation. Compared to the bulk carbon nitride, the caffeine‐doped analogue possesses a higher photocatalytic activity for the degradation of rhodamine B dye. Furthermore, this approach is also shown to be suitable for the modification of carbon nitride electrodes.

Enantioselective Nanoporous Carbon Based on Chiral Ionic Liquids

One of the greatest challenges in modern chemical processing is to achieve enantiospecific control in chemical reactions using chiral media such as chiral mesoporous materials. Herein, we describe a novel and effective synthetic pathway for the preparation of enantioselective nanoporous carbon, based on chiral ionic liquids (CILs). CILs of phenylalanine (CIL(Phe)) are used as precursors for the carbonization of chiral mesoporous carbon. We employ circular dichroism spectroscopy, isothermal titration calorimetry (ITC), and chronoamperometry in order to demonstrate the chiral nature of the mesoporous carbon. The approach presented in this paper is highly significant for the development of a new type of chiral porous materials for enantioselective chemistry. In addition, it contributes significantly to our understanding of the structure and nature of chiral nanoporous materials and surfaces.

Synthesis of novel 2-d carbon materials: sp2 carbon nanoribbon packing to form well-defined nanosheets

The conversion of simple glucose in a salt flux results in functional carbon materials which contain larger quantities of N and S as dopants. This “salt and sugar” approach gives access to a new type of mesostructure, where single carbon ribbons terminated with oxygen and sulfur or nitrogen functionalities are “knitted” towards very homogeneous, about 10 nm thick layers with very large specific surface areas of up to 3200 m2 g−1. Aberration corrected high resolution TEM together with EELS reveals the details of this structure.

Functional porous carbon nanospheres from sustainable precursors for high performance supercapacitors

Functional porous carbon nanospheres with tunable textural properties and nitrogen functionalities were synthesized from a cheap and sustainable phenolic carbon precursor (tannic acid) and nitrogen precursor (urea) using a facile one-step salt-templating method. The diverse functional carbons were obtained by calcination of mixtures of different molar ratios of urea to tannic acid (0 : 1, 5 : 1, 9 : 1, 13 : 1 and 17 : 1) with a eutectic salt (NaCl/ZnCl2) that was used as the porogen. The physico-chemical characterization of the obtained microporous carbons demonstrated that the textural properties, morphology, surface functionalities, and conductivity were strongly influenced by the molar ratio of urea to tannic acid. The nitrogen content in the carbons increased with the molar ratio of urea, reaching a maximum of 8.83% N at the highest molar ratio while the specific surface area (SBET) of the microporous carbons varied from 890 m2 g−1 to 1570 m2 g−1 depending on the synthesis conditions. The electrochemical performance of the carbon nanospheres in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI) was also significantly influenced by the synthesis conditions due to the combined effect of textural properties, morphology, nitrogen functionalities and electrical conductivity. Supercapacitors based on the functional porous carbon synthesized with a molar ratio of urea to tannic acid of 9 : 1 exhibited the best performance, with a specific capacitance as high as 110 F g−1 and a real energy density of 33 W h kg−1, when charged–discharged at 3.5 V in PYR14FSI.